1. Field of the Invention
The present invention generally relates to a radio line design of a mobile communication system, and specifically relates to a power calculation method used by the radio line design employed when defining a service area (cell radius) and a transmission power of a base station of the mobile communication system.
The present invention further relates to an apparatus that can perform the radio line design according to the power calculation method.
2. Description of the Related Art
In a conventional mobile communication system such as a PDC (Personal Digital Cellular) system, a mobile communication service is provided by dividing a service area into a plurality of relatively small zones, called cells. This type of mobile communication system provides mobile communication service, such as voice communication, by establishing radio channels between a plurality of base stations and a plurality of mobile stations.
In such a mobile communication system, radio wave transmitted from a transmitting point, i.e., a base station or a mobile station, reaches a receiving point, i.e., another mobile station or another base station after attenuation through the space. The grater Magnitude of the attenuation becomes, the larger a range between the transmitting point and the receiving point gets. The receiving point requires certain minimum signal strength in order that a signal is restored in predetermined quality, as defined by, for example, a BER (Bit Error Rate) and a BLER (Block Error Rate). Where a transmission power of the transmitting point is a fixed value, there is a limit in the range for a signal to reach the receiving point at a satisfactory strength level. The range can be increased by installing a higher transmission power amplifier, and by improving receiving performance. In addition, employing an antenna of a higher gain, and shortening a cable between a transmitter and the antenna, reducing loss, can extend the range. While these variables determine a maximum service-able range for a given transmission power, how much transmission power is required to attain a given range can also be obtained, making the transmission power a variable. To structure an efficient service area in the mobile communication system using radio lines, it is highly important to obtain an optimum relation between the service-able range and the transmission power.
In this radio line design, in order to examine the relation of the range of a signal and transmitted power, a table such as shown in FIG. 4 is generally used. The table of FIG. 4 is set to an uplink (from a mobile station to a base station) of a CDMA (Code Division Multiple Access) mobile communication system, for example. Each of entries (a) through (t) of this table is inter-related by formulas. If a transmission power, for example, is inputted, a service-able range corresponding to the transmission power is computed. If a desired range, to the contrary, is inputted, a transmission power required to obtain the range will be computed.
Such a table is realizable with various kinds of spreadsheet software that generally operates on a personal computer. The table of FIG. 4 includes a transmission power (a), a transmission antenna feeder loss (b), a transmission antenna gain (c), a receiving antenna gain (e), a receiving antenna feeder loss (f), a building penetration loss (q), and the like, which are taken into consideration as losses and gains of propagation. A required receiving power (m) is a function of a total noise (j), information transmission speed (12.2 kbps (1), 64 kbps (2), 384 kbps (3)) (k2), and required Eb/(N0+I0) (1). Here, the total noise (j) is a sum of a noise figure (NF) of a receiver (g), a thermal noise power density of the receiver (h), and an interference margin (i). In this example, a diversity handover gain (diversity gain by a simultaneous connection with a plurality of base stations) (n) that is characteristic of CDMA mobile communication, a shadowing margin (o) by buildings, and a fluctuating transmission power margin due to high-speed transmission power control (p), etc., are also considered. Here, the interference margin (i) is a margin allowed for interference caused by other users' communications to signal transmission of the receiver concerned in the case of CDMA mobile communication. The interference power is considered in the radio line design in this manner. In this table, properties are given in dB values such that evaluation is realized by adding and subtracting. Generally, this table is called a link budget, and widely used in a radio line design.
As mentioned above, a similar technique to the conventional radio line design, i.e., link budget, is applicable to the radio line design of the uplink circuit in the CDMA mobile communication, by introducing the interference margin representing an amount of the uplink interference that can have big influence on capacity. However, the conventional link budget technique cannot be applied as it is to the downlink of the CDMA mobile communications (called CDMA hereafter) for the following reasons.
In the downlink of CDMA, signals are transmitted from a base station to a plurality of mobile stations in the same radio frequency band. In order to increase receiving signal strength at a mobile station in a cell, an area of the cell can be set small, that is, a smaller radius cell helps providing relatively stronger signal to the mobile stations in the cell. However, for a mobile station that is nearby the base station, and therefore requires a smaller output of the base station, a relatively strong signal to a relatively remote mobile station causes interference. In this manner, making the cell radius smaller does not provide a desirable solution in terms of a ratio of signal strength of a desired signal vs. interference signals. In other words, a total output power transmitted from the base station is a source of the interference against mobile stations.
Further, similar interference is conceivable from other cells, as explained with reference to FIG. 3.
FIG. 3 shows two base stations 200 and 210, each transmitting at an output power level of 100. Each of mobile stations 100 and 110 near boundary of a respective cell is receiving signals from the base stations 200 and 210, respectively, at a receiving signal strength level of 5. In an attempt to increase the receiving signal strength level, suppose that a cell radius is decreased as shown by (b) of FIG. 3. Certainly, the signal strength level of a desired signal received at a mobile station 120 is increased to 20. However, the signal strength level of an undesired signal from a base station 230 is also increased to 20. In terms of the ratio of the desired signal vs. interference, there is no improvement.
As above, there are two important parameters in designing a CDMA downlink circuit, namely, a total output power of a base station and how much of the total output power to be assigned to a design target channel. Studies on this point have been published. For example, reference 1 (Ishikawa, Nakano, Uebayashi, “DS-CDMA Mobile Communication Downlink Radio line Design Method”, the Institute of Electronics, Information and Communication Engineers 1997, Society Conference, B-5-8, September, 1997) analyzes in detail how the total power of a base station and the ratio of the power assigned to a design-target channel influence receiving quality, such that an optimum total power and an optimum ratio are obtained. Reference 2 (Hayashi, Usuda, Ishikawa, Nakamura, Onoe, “Study on Power Ratio to a common control channel in a downlink of W-CDMA”, the Institute of Electronics, Information and Communication Engineers 2000, General Conference, B-5-81, March, 2000) provides a detailed report on how distribution ratio of the power influences a BER of a channel.
As described, the CDMA downlink circuit has a complicated property that receiving signal strength of not only a desired signal, but also interference of other station, significantly varies with a cell radius and various losses. For this reason, design of a downlink circuit has been performed by computer simulation, etc., that is, the conventional link budget technique, in which calculations are based on adding and subtracting a gain and a loss, cannot be applied as it is.